This invention relates to an apparatus and method for control of gaseous emissions of ammonia (NH.sub.3), oxides of nitrogen (NO.sub.x) or volatile organic compounds (VOC) from various processes where these compounds are used, present or formed.
NH.sub.3, NO.sub.x or VOC are frequently used in industrial processes as reactants or they may be formed as a result of chemical or biochemical reactions. These compounds, which are frequently vented from the process, can adversely impact air quality. Emissions of these compounds are, therefore, frequently regulated by the government and controlled by application of a variety of technologies. Some of the most frequently used technologies utilize water, sodium hydroxide or sulfuric acid scrubbing processes.
In the case of water scrubbing, there is a limitation resulting from the water becoming saturated with the dissolved pollutant. This inhibits further absorption of the pollutant by the water. In many situations, water solutions of sulfuric acid or sodium hydroxide are used as the scrubbing solutions to improve the solubility of the pollutant. However, the scrubbing solution may still become saturated with the dissolved pollutant inhibiting further absorption.
In the present invention, the scrubbing solution contains biological media which metabolizes the pollutant at a rate such that the scrubbing solution does not become saturated with the absorbed pollutant. As the absorption rate of the pollutant by the scrubbing solution is proportional to the difference between the concentration of the pollutant in the gas phase and the absorbed phase, this invention allows for high absorption rates of gaseous pollutants.
A prior art biochemical process, used to control emissions of vapors of VOC, is based on an attached growth bioreactor model where a reactor is filled with packing such as for example granular activated carbon which in turn promotes biological growth on the surface of the carbon. The gases pass around the packing covered with the biological growth and are absorbed and treated.
Unlike this prior art process, the present invention utilizes suspended growth of the biological process rather than attached growth. This difference results in a reduced sensitivity to pollutant concentration fluctuations in the process gas stream entering the bioscrubber. This is accomplished in the invention by maintaining an inventory of microorganisms in the bioreactor section of the bioscrubber and adjustment of the availability of the microorganisms to treat the gaseous emissions.
Aqueous suspensions containing microorganisms have been used in the past to treat waste gas streams. For instance, U.S. Pat. No. 4,723,968 to Schippert, et al. discloses a method and apparatus for the purification of waste air containing biologically decomposable impurities while U.S. Pat. No. 1,701,825 to Seil discloses a gas purification process that uses certain bacteria to treat a gas stream. Both of these references, however, are limited to treating specific compounds. Further, as will be made apparent from the following description, various features and aspects of the present invention remain absent from the prior art.